Since a fiber-reinforced composite comprising a fiber and a matrix resin is light and excellent in chemical property, it has been applied in various uses in recent years. With expansion of use, a fiber-reinforced composite is required for various physical properties, and one of them is improvement in heat resistance.
As a heat resistant fiber-reinforced composite, a composite using a polyimide resin as a matrix resin is known (see. JP-A No. 5-9312 (page (2), lines 34 to 39)). A heat resistance temperature of a polyimide resin is about 300° C. In order to further improve heat resistance, use of a ceramic matrix is also known (see. JP-A No. 9-100174 (claim 1, etc.)). A heat resistance temperature of a ceramic matrix is about 1000° C. In addition, as a fiber-reinforced composite excellent in heat resistance, there is also a carbon fiber-reinforced composite. A carbon fiber-reinforced composite is obtained by burning a carbon fiber-reinforced plastic in inert atmosphere at a high temperature, and carbonizing a plastic as a matrix (see JP-A No. 6-191943 (claim 1, etc.)).
However, when a polyimide resin is used as a matrix resin, there is a problem that a melting temperature of a polyimide resin is high. When a melting temperature is high, it is difficult to adhere an interface between a fiber and a matrix resin due to a difference between a thermal expansibility of a matrix resin and a thermal expansibility of a fiber. In addition, there is also a problem that, in order to cure a polyimide resin, a high temperature is required. When a curing temperature of a matrix resin is high, there is a possibility that oxidative deterioration of a fiber is caused, and a further heat resistant apparatus becomes necessary, and energy efficacy at molding is also worse. Further, there is also a problem that a polyimide resin is very expensive.
A fiber-reinforced composite and a carbon fiber-reinforced composite using a ceramic matrix have a defect that they are excellent in heat resistance, but are fragile. In addition, a carbon fiber-reinforced composite also has a problem that a manufacturing step is complicated, and manufacturing is laborsome.
On the other hand, in a heat-resistant fiber-reinforced composite, as a reinforcing fiber, a heat-resistant fiber is used, a representative which is an aramid fiber. However, the aramid fiber has a problem that it has not necessarily high adherability with other organic polymers. Besides, the aramid fiber simultaneously has natures which are not preferable depending on use purpose, such as high equilibrium moisture content of a fiber (for example, an equilibrium moisture content of aramid fiber composed of a para-system homopolymer is about 7%), easy deterioration with ultraviolet-ray, and easy fibrillation in a certain type.
Various modification proposals have been made for compensating for these disadvantageous natures. For example, a method of modifying a polymer structure itself by copolymerizing a third component of an ether system, a method of adding a modifying substance to a spinning dope at spinning, a method of etching a surface of the resulting fiber with excimer laser, a method of injecting a metal by an ion beam, and a method of introducing a modifying substance into a fiber using a supercritical fluid have been previously proposed.
However, in the method of copolymerizing a polymer itself, although some extent of a low equilibrium moisture content and prevention of fibrillation can be realized, improvement of other functions can not be expected, and there was a problem of, for example, deterioration in heat-resistant dimensional stability. In addition, in the method of adding a modifying substance to a spinning dope, there was a problem that a substance to which the method can be applied is extremely limited due to speciality of a spinning solvent, and only coloring with a pigment can be industrially realized. Effect of improving adherability is insufficient even by etching of a fiber surface with excimer laser, treatment with a supercritical fluid is still in a region at a laboratory level, and a range of substances which can be introduced was limited. Like this, all methods have not sufficient effects, and are restrictive means for a limited purpose, and a modifying method leading to improvement in general-use function has not been known.
Various heat-resistant fibers other than aramid fiber have problems, a part of which is common with the aforementioned problems of aramid fiber. And, also in these fibers, a modifying method leading to improvement in general-use function has not been known.